This invention relates to an apparatus for reducing aerodynamic background noise in the test section of a transonic wind tunnel. Since the development of the transonic wind tunnel during the early 1940's, an almost continued effort has been made to improve air flow quality. During the early development years, it was observed that wind tunnel blockage and reflected shock waves from the models being tested would seriously affect the determination of loads and other aerodynamic data. The wind tunnel blockage phenomenon consists of standing shock waves in the tunnel test section and problems associated with startup of the wind tunnel. The problem of reflected shock waves consists of shock waves originating at the model reflecting from the wind tunnel walls back to the model for influencing any data being acquired from the air flow around the model.
The utilization of porous walls was one technique developed to reduce the problems of wind tunnel blockage and shock wave reflections. In this technique, suction could be applied to the porous wall which aided in starting the wind tunnels and for maintaining the air flow through the test section in a uniform manner. Later, this technique was refined so as to optimize air flow conditions for all mach numbers by providing a variable porosity wall surface for the test section of the wind tunnel, with the variable porosity wall opened to a setting that would provide optimum wave cancellation for a particular transonic mach number and also eliminate the problems of blockage.
Even though the variable porosity walls have improved the flow quality in wind tunnel test sections in some manner, evidence exists which indicates that the porous wind tunnel walls also produce high background noise levels in wind tunnel test sections. For the most part, fluctuating pressure data collected from experiments conducted in transonic wind tunnels have been incorrect or in some cases completely drowned out by the high background noise levels of the wind tunnel. It has been found that the perforations or holes formed in the porous wind tunnel walls subject the air stream to the sharp edges and corners of the perforations with the walls at any porosity setting between zero and maximum and that the sharp edges of the hole create a whistling effect of the air passing thereover. The sharp edges and corners create edge tones, and shock and expansion waves that are radiated into the free stream which influence not only the fluctuating pressure measurements on the model, but also the steady state flow conditions in the wind tunnel test section. Therefore, to increase the accuracy and reliability of the aerodynamic data taken in transonic wind tunnels, it is necessary to suppress the background noise created by the porous walls.